This invention relates to a magnetic storage system for coded numerical data with code conversion into and from high density bipolar code of order n.
There is a problem involved in the high-density recording on magnetic tape of numerical data supplied for example by a sonar system.
This problem is primarily to obtain a recording-reading cycle for a maximum flow of numerical data allowing for the band width of the recording system. To this end, it is necessary:
on the one hand to avoid delivering to the recorder signals having a continuous component which the recorder would be unable to restore;
on the other hand to be able to carry out reading at a clock frequency without which, the numerical signals recorded could not be decoded, and
in addition to obtain a recording-reading cycle with a minimum of transcription errors whilst, at the same time, avoiding the need for complex and sophisticated hardware.
The solution to the problem provided by the invention in the field of recording signals of the sonar type, may be directly transferred to other fields, involving the high-density recording of numerical signals, particularly to the data processing side of telecommunications and radar and to the field of space.
Various codes are used by specialists, of which the principal representatives are known in abbeviated from as NRZ (non-reset to zero), RZ (reset to zero), MILLER, BI-PHASE and their derivatives.
These codes have advantages and/or disadvantages which, in the field of magnetic recording, are primarily associated with the band width of the spectrum of the numerical code, the central frequency of that spectrum, the transmission of low frequencies and, in particular, of the continuous component, and with the difficulty of restoring the signals to the clock frequency required for decoding during readout.
Thus, the disadvantage of an excessively wide band width is encountered in the RZ and BI-PHASE codes, the disadvantage of an excessively high central frequency is encountered in the BI-PHASE code, the disadvantage of the existence of a continuous component is encountered in the NRZ and RZ codes and, to a lesser extent, in the MILLER code and the disadvantage of being unable to restore the clock signals during excessively long sequence of "1" or excessively long sequences of "0" is encountered in the NRZ and RZ codes.
Artifices are used for overcoming the various difficulties, giving rise to other codes such as, for example, the enhanced NRZ code, the scrambled NRZ code and the square MILLER code. They use complex, expensive and unstable circuits which lack flexibility or fail to attain the performance expected of them.
The various codes to which reference has so far been made are binary codes, i.e. codes having only two levels "0" and "1" or, which amounts to the same, -1 and +1.